求教：LSAT-7-1-10

10.   A large group of hyperactive children whose regular diets included food containing large amounts of additives was observed by researchers trained to assess the presence or absence of behavior problems. The children were then placed on a low-additive diet for several weeks, after which they were observed again. Originally nearly 60 percent of the children exhibited behavior problems; after the change in diet, only 30 percent did so. On the basis of these data, it can be concluded that food additives can contribute to behavior problems in hyperactive children.

The evidence cited fails to establish the conclusion because

(A) there is no evidence that the reduction in behavior problems was proportionate to the reduction in food-additive intake

(B) there is no way to know what changes would have occurred without the change of diet, since only children who changed to a low-additive diet were studied

(C) exactly how many children exhibited behavior problems after the change in diet cannot be determined, since the size of the group studied is not precisely given

(D) there is no evidence that the behavior of some of the children was unaffected by additives（B）

(E) the evidence is consistent with the claim that some children exhibit more frequent behavior problems after being on the low-additive diet than they had exhibited when first observed

答案：B，不明白为什么E不对？

谢谢！

我的理解是这样的:

这是一道因果推理题, 题目中researcher用的方法是"求同法", 即对一些患有相同疾病的人进行研究, 然后通过比率降低说明原因是large amounts of additives. 这种方法的最大问题是"不完全归纳", 因为虽然是a large group, 但不是whole. 所以这样的因果推理存在错误的可能. 削弱可能有几种方式: 1) 直接指出它因进行削弱; 2) 指出survey的范围不够大, 没有代表性或不充分; 3) 方法的局限性, b中给出了另外一种补充方法,即"求异法". 两种方法并用, 可以更加有说服力.

所以我不认为b是它因削弱, 而是对方法不科学的削弱.

a 违背了原题, 原题中的试验是一个evidence;

c 为out of scope, 题目中已经给出ratio, 而且ratio有说服力, 不用涉及absolute number.

d为irrelevant, 论题说的是 additives的影响,

e违背了原题意思, 原题中给出了the evidence is consistent with the claim that some children exhibit less frequent behavior problems after being on the low-additive diet than they had exhibited when first observed,  

个人理解, 请指教!

谢谢几位大侠的指点，现在明白了B为什么对。

但还是不大明白为什么E不对。原文说的是人数量的减少，而E指出behavior problem度的增加，所以我认为可以驳behavior problem与additive有关。

哎，进了牛尖里，不知怎么出来，还请继续指点出路。谢谢！